Hard materials such as diamond, cubic boron nitride and wurtzite boron nitride are widely applied to tools such as cutting tools, wear-resistant components, abrasive grains and the like due to the high hardness of these materials. These materials are also applied to heat radiation substrates for semiconductor elements due their to high thermal conductivity. Further, research has been made in order to apply these materials to optical elements and semiconductor materials for effectuating the light transmissivity thereof.
These materials, which are stable under high pressure as high pressure phase materials, were initially artificially synthesized only under superhigh pressure. Various methods have been recently developed for synthesizing such hard materials, including diamond, from decompressed vapor phases. Among such methods of vapor-phase synthesizing hard materials, a microwave plasma CVD process of generating plasma by microwaves for decomposing gas and depositing a hard material on a substrate is an excellent method which can synthesize a hard material of high purity. Japanese Patent Laying-Open Gazettes Nos. 58-110494, 59-3098 and 59-63732, and U.S. Pat. No. 4,434,188 disclose methods for synthesizing hard materials through the microwave plasma CVD process.
FIG. 1 shows a conventional apparatus for vapor-phase synthesizing diamond through microwaves. This apparatus will be now schematically described.
Referring to FIG. 1, microwaves oscillated by a magnetron oscillator 1 are applied to a substrate 8, which is placed on a support 7 provided in a reaction tube 6, through an isolator 2, a power monitor 3, a tuner 4 and a waveguide 5. Raw material gas, which is prepared by mixing methane gas, hydrogen etc. at a prescribed ratio, for example, is introduced through a gas inlet port 9 and exhausted from an exhaust port 10 by a pump (not shown) The discharge of the raw material gas is so adjusted that a prescribed pressure is maintained in the reaction tube 6. Plasma 11 is generated by the energy of the microwaves, to form diamond on the substrate 8. A cooling water supply tube 13 supplies an applicator 12 with cooling water, which in turn is discharged from a cooling water discharge tube 14, in order to prevent an excessive heating of the reaction tube 6.
The waveguide 5 implements optimal conditions of compatibility by movement of a plunger 15 or adjustment of the tuner 4 in response to the wavelength of the introduced microwaves.
As shown in FIG. 1, the raw material gas is introduced into and exhausted from the apparatus along arrows A and B respectively.
Japanese Patent Laying-Open Gazette No. 61-174378 or the like also discloses a method of synthesizing cubic boron nitride through such a microwave plasma CVD apparatus.
In general, electromagnetic waves in a frequency domain or range of 300 MHz to 300 GHz are called microwaves. The electromagnetic waves of this frequency domain are easily absorbed by molecules, and have wavelengths of 1 m to 1 mm, which are short compared to the size of a reaction tube. Thus, it is possible to efficiently generate plasma in gas under a wide range of pressure. Further, the microwaves of this frequency domain can efficiently supply electric power through a waveguide having a practical size of not more than scores of centimeters, with smaller problems of loss and leakage in a supply path of a coaxial cable etc., dissimilarly to waves of a lower frequency domain.
However, a method of and an apparatus for synthesizing a hard material through the aforementioned conventional microwave plasma CVD apparatus have the following problems:
While microwaves of 2.45 GHz in frequency are widely industrially employed, standing waves are caused in a cycle of a half-wave length thereof if the microwaves are trapped in a reaction tube. Thus, plasma is heterogeneously generated in response to a strength distribution of the standing waves. Therefore, it is impossible to homogeneously generate plasma over a wide area for synthesizing a hard material.
The microwaves have such a property that the same can stably generate plasma under a higher gas pressure and improve the growth rate of the hard material as the frequency thereof is increased. When the frequency of the microwaves is thus increased, however, the wavelength is shortened whereby areas where large amplitude portions of the standing waves occur are reduced.
In order to improve such growth heterogeneity in the vapor synthesis of a hard material, there has been reported a method of applying a dc magnetic field for controlling the streamline of plasma and a method of reducing the gas pressure in a reaction tube in Japanese Patent Laying-Open Gazette No. 63-107899. In any of these methods, however, mean free paths of active molecule species and electrons contained in the plasma must be kept long. Thus, the gas pressure in the reaction tube must be reduced to not more than 10 Torr, and the hard material is grown at an extremely slow rate of less than 1 .mu.m/h.
Thus, the conventional method of synthesizing a hard material such as diamond, by the plasma CVD process has been significantly restricted in growth area and growth rate, and it has been difficult to mass-synthesize the hard material.